COASTAL GARDENER
Franklin Laemmlen, Ph.D.
Q:
I have a severe whitefly infestation in my backyard. I have tried every spray in the
book. The infestation has not been controlled and, in fact, seems to be getting worse.
Can you help?
A:
First a little information about whiteflies. There are several species present on the
Central Coast. Ash whitefly, citrus whitefly, and the greenhouse whitefly (GW). Of
the three, the latter is most common and the biggest problem for homeowners. One
female GW can lay several batches of 30-40 eggs in her lifetime. Development from
egg to adult takes 12-15 days depending on the temperature. Both nymphs and adults
feed by sucking sap from the plant. In this process, they excrete honeydew, which
makes plant surfaces sticky and shiny at first, then sooty mold fungus grows on the
honeydew, and the leaves turn sooty black. The presence of ants can exacerbate the
whitefly problem. Ants collect honeydew, and they will protect their food source by
harassing predators and parasites that come to attack the whiteflies. Whiteflies
reproduce rapidly. They also have a notorious ability to develop resistance to plant
protection chemicals. Therefore, there are not many products which provide
satisfactory long-term control of GW. Considering these factors, whitefly control
should center around maintaining a balance between whiteflies and their natural
enemies. If ants are present, they should be controlled. Ant colonies should be
suppressed by using baits or granules, which will control ants at their source (colony
site) rather than in the plant where they are collecting honeydew. Next, try to keep
whitefly sprays to a minimum. Many plant protection products will kill predators and
parasites as well as the whiteflies, and the whiteflies tend to recover faster than the
insects of prey. If you must spray, use summer oil and/or insecticidal soap to reduce
populations. These products are less harmful to predators and parasites. Second, there
are several biological control supply companies on the Central Coast from which
parasites and predators can be purchased. Parasites must be used as adults, however,
predators should be purchased as eggs or larvae, so they have to remain and feed
where they are placed. Often adult predators will disperse from a deposition site
before effective control is established. Finally, use yellow sticky tape. This product is
usually made of yellow plastic, which has been coated with tanglefoot adhesive.
Whiteflies are attracted to yellow and, of course, will be entangled in the adhesive. A
product, called “HopperFinder” or just ask for “sticky tape,” to catch insects can be
purchased at several farm supply stores on the Central Coast. Remember, whitefly
management is not a one-time event. Whitefly predators and parasites will most
likely have to be released several times to affect suppression of a population. The
yellow sticky tape will become coated with whiteflies and will have to be replaced
periodically. Use disposable gloves when handling the sticky tape. If you get
adhesive on yourself or your clothing, WD-40 is an effective solvent. Wrap used
sticky tape in plastic or paper for disposal.
For more information about whiteflies contact The Coastal Gardener, 624-A West Foster
Road, Santa Maria, CA 93455.